1. Field of the Invention
The present invention relates to a control of brake force or driving force for preventing a reverse motion/forward motion of a vehicle at the time of uphill/downhill starting.
2. Description of Related Art
In an automatic transmission having a torque converter, even with a throttle fully closed in the drive range, torque from the torque converter that is being driven by the engine which is running at idling speeds is transmitted to the vehicle, whereby the vehicle can creep. On the other hand, in automatic transmissions which has no torque converter but includes a CVT, an automatic manual transmission and a two-pedal (a clutch pedal-less) transmission, in many cases a control is implemented such that creep is enabled with the throttle fully closed in the drive range by the clutch being partially engaged.
When a vehicle comes to standstill on a slope, if a driver removes a pedal effort to allow for returning the brake pedal, brake hydraulic pressure of a master cylinder is decreased and the vehicle moves backwards.
To prevent this backward moving, the brake force needs to be changed in response to a change in driving force arising as the accelerator pedal is depressed without relying upon the brake hydraulic pressure of the master cylinder or the driving force needs to be changed without relying upon the depression of the accelerator pedal until the accelerator pedal is actually depressed. At this time, since a slope component of gravity acceleration differs depending upon an inclination angle of a slope, and brake force or driving force necessary to stop the vehicle is changed. Thus, the brake force or driving force needs to be controlled in accordance with the inclination angle of a slope. For this reason, the inclination angle of a road surface needs to be measured.
In many cases, a longitudinal acceleration sensor is installed to measure an acceleration at the time of braking, and its capacity is, for example, 1.5˜2 G. On the other hand, for an example, the acceleration component which is generated on the road surface having the inclination angle of 1° is substantially G×sin (1°)=0.0175 G.
That is, the acceleration component at inclination angle of 1° corresponds to 1% of full scale of the longitudinal acceleration sensor. Therefore, if the installed longitudinal acceleration sensor, the longitudinal acceleration sensor is used at a lower limit of its resolving power. Therefore, it cannot be expected that the longitudinal acceleration sensor measure a road surface inclination with high accuracy.
In addition, it is costly to install a highly accurate sensor exclusively for measuring such an inclination angle. Additionally, since an inclination sensor is attached to a body, an additional separate unit becomes necessary for detecting the inclination of the body relative to the road surface in order to measure a road surface inclination angle accurately.
There is Japanese Patent Examined Publication JP-B-3598491 which discloses a technique related to preventing the backward movement of a vehicle at the time of slope starting. JP-B-3598491 describes that the vehicle is prevented from moving backwards in such a way that when the vehicle speed reaches 0 Km/h, even with an engine stopped, the brake hydraulic pressure is held so as to keep brake force exhibited, while when a brake pedal is released and the engine is automatically started, the brake force is gradually reduced.
However, the related art has the following problems since the brake force is not controlled or the driving force is not controlled as the pedal effort is removed to allow for the return of the brake pedal. When driving force is exhibited by depressing the accelerator pedal after the brake pedal has been released, in the event that the brake force is released too early, a sufficient driving force has not yet been exhibited at a point in time at which the brake force is released, and the slope component of the gravity acceleration becomes large compared with the driving force available then, leading to a problem that the vehicle is forced to move backwards.
In contrast, if the brake force is released too late, the drive force becomes equal to the slope component of the gravity acceleration and even if no brake force is provided, the brake force remains even at a point in time at which the vehicle is stopped, leading to a problem that brake dragging is generated.
Furthermore, if the brake force is controlled based on the inclination angle of a road surface, when an additional exclusive sensor for measuring an inclination angle is provided, this leads to a problem that costs are increased which are incurred in relation to prevention of the backward movement of a vehicle at the time of slope starting. In addition, if the longitudinal acceleration sensor is used to calculate an inclination angle, an accuracy problem is caused, and the backward movement of the vehicle cannot be prevented with high accuracy.
Since JP-B-3598491 is not such that the brake force is controlled in accordance with driving torque and the slope component of gravity acceleration, there were similar problems to those described just above.